This invention relates to an instantaneous exposure control for light sensitive films by terminating the exposure after the light sensitive substance on the film has been subjected to the quantum of light which will yield the desired film contrast levels without regard to the actual exposure times, light source intensities, etc.
When photographic images are made the quality of the image is determined by its sharpness, which is the function of the optics and their proper adjustment, and by the amount of light to which the film is exposed. The latter primarily determined the contrast, namely, the density or blackness and the transparency of certain portions of the film and intermediate gray scales of other portions of the film. Each film has an optimal exposure value, which for convenience, can be called an optimal quantity of light to which it is exposed. Insufficient or excess exposure will reduce the quality of the image and, therefore, of the finished film.
One particularly critical application of photographic film is in the area of high speed, large volume duplication of photographic records, such as records stored on microfiche masters which is to be transferred onto microfiche copies or copy film. For a number of reasons, such as its greater resolution, versatility, storing qualities, etc. vesicular film is used in an ever increasing amount for such applications.
To briefly summarize the characteristics of vesicular film, light sensitive diazonium salt is uniformly disposed in a thin thermoplastic resin layer carried by a conventional transparent substrate. Upon exposure to light, particularly ultraviolet radiation, the diazonium salt decomposes and releases nitrogen and other volatile fragments which collect in small nucleating centers. Upon subsequent heating, the nuclei expand to form microscopic vesicules or bubbles which scatter incident light and thus form an image. The heating step is analogous to the developing of conventional silver film.
Vesicular film copies are normally made by contact printing the copy film from a microfiche master, that is by placing the copy film into intimate contact with the master film and directing light from a light source through the master onto the copy film. In the past, actual exposure times were determined by trial and error. First, the exposure times were set by experiences, that is the operator sets the exposure time to what he believed would yield a copy fiche with the desired image density or blackness. After the first copy had been made it was inspected and if it was too light or too dark the exposure time was suitably adjusted until the desired image quality was attained.
This system is time-consuming, can waste a substantial amount of expensive film and still permits a fair amount of deviation based on the operator's subjective impressions, observations and his interpretation of what the image should look like. However, since the copies are frequently used on sensitive equipment, image uniformity is highly desirable.
In addition, the prior art approach of relying on the operator's judgment to obtain optimal exposure control does not take into account variations in the film exposure due to changes in the light intensity from such factors as changes in the line voltage, reflector contamination, lamp aging, etc., changes in the light sensitivity of different copy film batches, base and fog variations of the master film, and the like. Nevertheless, such changes and variations affect the quantum of light that reaches the copy film during a given time interval, or the quantum of light required for exposing the copy film to the desired level. Thus, unless the copies are constantly monitored, which is frequently impossible due to the high copy volume, the quality of the copy film can at best be maintained within a more or less acceptable range, but not at peak value.